In pending U.S. application Ser. No. 08/620,983, filed Mar. 22, 1996, abandoned, supra, there is disclosed base oils which are non-toxic, non-polluting, biodegradable and suitable for use in forming drilling fluid compositions by dispersing therein at least one additive selected from the group consisting of weighting agents, emulsifiers, wetting agents, viscosifiers, fluid loss control agents, proppants and other particulate agents. In forming the drilling fluids a continuous oil phase, or water-in-oil invert emulsion phase (where water or brine is present and emulsified in the solvent), provide a medium for the dispersed additive, or additives. The base oil is constituted of a mixture of normal paraffins (n-paraffins) and isoparaffins having from about 10 to about 24 carbon atoms, i.e., C.sub.10 -C.sub.24 n-paraffins and isoparaffins, preferably C.sub.12 -C.sub.18 paraffins, having an isoparaffin:n-paraffins molar ratio ranging generally from about 0.5:1 to about 9:1, and the isoparaffins of the mixture contain greater than 50 percent, generally 70 percent, by weight of the isoparaffins in the mixture, of mono-methyl species, e.g., 2-methyl, 3-methyl, 4-methyl, .gtoreq.5-methyl or the like, with minimal formation of isoparaffins having branches with substituent groups of carbon number greater than 1, e.g., ethyl, propyl, butyl or the like.
These base oils form high performance drilling fluids of low toxicity, and they are environmentally friendly. Like conventional diesel fuels they form drilling fluids, or muds, which have excellent rheological properties for rotary drilling, but unlike conventional diesel fuels do not pose a serious and costly disposal problem for offshore operators. Thus, with conventional diesel operators must barge spent mud and cuttings to shore for land disposal, because diesel fuel cannot meet U.S. Environmental Protection Agency Standards regarding discharge limitations and/or permit requirements. In contrast, this new base oil admirably meets this need, and provides excellent Theological properties. However, the oil and drilling fluids made from the new base oil is lacking in that it is of low pour point, and its low temperature performance leaves much to be desired. It does not perform well under cold temperature conditions which are frequently encountered in drilling operations, e.g., in Alaskan or North Sea operations. Accordingly, there remains a need for a further improved base oil; and drilling fluid made from such base oil, of low toxicity, biodegradable, and environmentally friendly, but yet will be useful at low temperature conditions.